Exoplanets and the search for life in the cosmos

EXOPLANETS, planets that orbit stars other the sun, are being discovered at a tremendous rate – upwards of three thousand in the last decade alone. The prospect of finding life beyond our own solar system is an enticing one, and a lot of money and research is being channelled into the endeavour. One recent discovery has got the scientific community particularly hot and bothered.

On 22 February, NASA announced the discovery of seven Earth-si

Image: NASA/JPL-Caltech

zed planets packed tightly around TRAPPIST-1, a tiny star in our own cosmic backyard. The star – named for the Belgian telescope that carried out the initial research – is a miniscule and comparatively cool dwarf star which is around eight per cent the mass and 0.05 per cent the luminosity of the sun.

The alien worlds were found by looking for dips in the star’s infrared light output caused by one of the planets passing between the star and observers here on Earth – events known as transits. Astronomers can analyse these light dips and infer information about their respective orbits, sizes and even compositions. They found that three of the planets are comparable in both size and density to the Earth, which implies that they are also rocky in composition. The planets all complete their orbits between 1.5 and 13 days, meaning that they are incredibly close to their tiny host star.

What makes this discovery so enticing is these apparently very tight orbits position three of the planets within the ‘habitable zone’ of the star. The ‘habitable zone’ is the area around a star where an orbiting planet would receive just the right amount of heat to enable water to exist as a liquid – meaning conditions are just right to support life. Of course, the first question to ask is “can we go there?”.

Well, even though it is close in galactic terms – around 40 light years away – that is still too huge a distance for our feeble technology to surmount. Even at the speed of light it would take 40 years to get there, and the fastest spacecraft we currently have – the New Horizons probe – travelling at a fairly zippy 36 000 miles per hour would take the best part of a million years to get there. It’s a very

Image: wikipedia

long way. So traveling to the TRAPPIST-1 system is out of the question – at least for the time being – but that doesn’t mean we can’t pick up signatures of life from right here on Earth. By using spectroscopy (analysing the light from the star that passes through the planetary atmosphere), it is possible to determine the chemical composition of the atmosphere. Detection of key chemicals such as methane, oxygen and of course water would be tell-tale signatures of life.

Several new telescopes are being built with this exact challenge in mind – with the James Webb Space Telescope launch next year, finding the chemical fingerprints of water – and perhaps life – on these planets is a real possibility. “Answering the question ‘are we alone’ is a top science priority”, says Thomas Zurbuchen, NASA’s Associate Administrator of the Science Mission Directorate, “Finding so many planets like these for the first time in the habitable zone is a remarkable step forward toward that goal.”

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